Microbes in Human Welfare — NCERT Solutions

The key difference is:

• Primary treatment is mainly physical; it removes large and small particles by filtration and sedimentation.
• Secondary treatment is biological; useful aerobic microbes break down organic matter and reduce BOD.

Yes. Microbes can be used as a source of energy in the form of biogas. Methanogens break down dung and other bio-wastes anaerobically in biogas plants and produce a mixture of gases, mainly methane, which is used as fuel for cooking and lighting.

Microbes reduce the use of chemical fertilisers and pesticides in two ways:

• As biofertilisers: microbes like Rhizobium, Azospirillum, Azotobacter, cyanobacteria, and mycorrhizal fungi enrich the soil by fixing nitrogen or improving uptake of nutrients like phosphorus.
• As biocontrol agents: microbes such as Bacillus thuringiensis (Bt), Trichoderma, and baculoviruses control pests and plant diseases, reducing the need for chemical pesticides.

The most polluted sample is the one with the highest BOD, because higher BOD means more organic matter and greater oxygen demand. So the sample with 400 mg/L is the most polluted.

Assigning labels:

• Untreated sewage water → 400 mg/L
• Secondary effluent from sewage treatment plant → 20 mg/L
• River water → 8 mg/L

So the labels are:

• A = 20 mg/L = secondary effluent
• B = 8 mg/L = river water
• C = 400 mg/L = untreated sewage water

Decreasing order of importance:

Penicillin > Curd > Biogas > Citric acid

Reasons:

• Penicillin is placed first because antibiotics have saved millions of lives by controlling dangerous bacterial diseases such as plague, diphtheria and whooping cough.
• Curd is very important as a daily household product. It improves nutrition by increasing vitamin B$_{12}$ and also contains lactic acid bacteria (LAB) that help check disease-causing microbes in the stomach.
• Biogas is useful as a source of fuel for cooking and lighting, especially in rural areas, and also helps in waste management.
• Citric acid is useful industrially, but its direct importance for human welfare is less than the others listed above.

So, on the basis of direct impact on human health and daily life, penicillin is the most important, followed by curd, then biogas, and finally citric acid.

Biofertilisers enrich soil fertility by adding or improving the availability of nutrients:

• Rhizobium in root nodules of legumes fixes atmospheric nitrogen into usable organic forms.
• Free-living bacteria like Azospirillum and Azotobacter also fix nitrogen in the soil.
• Mycorrhizal fungi such as Glomus help plants absorb phosphorus from the soil.
• Cyanobacteria like Anabaena and Nostoc fix nitrogen and also add organic matter to the soil, increasing fertility.